One technique that is well known for fabricating preforms from which lightguide fibers are drawn is known as the Modified Chemical Vapor Deposition (MCVD) process and is described in detail in U.S. Pat. No. 4,217,027. In that process, the fabrication of a lightguide preform starts with a chemically cleaned, elongated fused quartz tube which is rotated in a glass lathe. A surface mixing oxy-hydrogen torch repeatedly moves along the length of the tube in one direction resulting in a hot zone moving therealong. Chemical vapors are directed into the rotating tube as the torch traverses its length, resulting in the deposition of a plurality of layers of fused, doped silica on the inside of the tube. The tube is then heated to an elevated temperature to collapse the tube with the layers therein resulting in a solid lightguide preform wherein the original glass tube becomes the cladding and the deposited layers form the core.
Most important to such a preform fabrication process is a highly concentric glass tube with minimum variation along its length. These are among the tube properties required to make them compatible with subsequent process steps and end product requirements. Drawing processes are commonly used to fabricate these tubes where their final shape is formed in free space under the guidance of drawing parameters.
It has been shown in an article titled "Extrusion of Fused Silica Cladding Tubes" by A. Coucoulas, in the TECHNICAL DIGEST of the Third International Conference on Integrated Optics and Optical Fiber Communication, page 112, April 1981 that extrusion techniques provide greater dimensional control of the tube than drawing processes. For extrusion makes use of an accurately machined die to control the final shape of the issuing extrudate. That article further describes a method and apparatus for fabricating fused silica tubes using a gas pressure head. The glass tube extruder therein described is comprised of an enclosed chamber having a crucible with a die therein which forms an annular opening in the bottom of the chamber, heating coils mounted about the lower portion of the crucible and means for applying a gas pressure within the chamber. A charge of silica glass is placed in the crucible and the coils are activated to heat the charge to a lower viscosity. A gas pressure is then applied to urge the glass extrudate through the annular opening as a tube.
Such a technique has been found to be most effective in extruding high quality glass tubes. However, some tubes have been found to be unacceptable based upon the stringent requirements placed on lightguide preform starting tubes which require that any tube of four feet in length having a bow with an arc height greater than one-eighth of an inch is rejected.
It has been found that during an extrusion run, the tube, at times, will bend or bow as it exits the annulus resulting in the rejection thereof. It appears that such bending is probably due to assymmetry in the temperature and/or the radial assymmetry of the annulus itself. However, it is most difficult to achieve a uniform temperature distribution about the annulus when using an inductive heating arrangement. Additionally, it is most expensive to fabricate extremely accurately dimensioned dies to provide the desired geometric symmetry.
A further problem associated with glass tube extrusion occurs near the end of the process as the desired length of the tube has been extruded. Usually of residue of solidified glass remains on the bottom portion of the crucible as the location of the inductive heating coils does not sufficiently heat that portion. Such solidified glass is most difficult to remove and can, at times, crack the crucible upon cooling.
Accordingly, there is a need to provide a technique for simply and effectively eliminating bowing of a tube during the extrusion process and for also eliminating the problem of the solidified glass residue.